1. Field of the Invention
The present invention relates generally to liquid heaters and particularly to swimming pool and spa heaters which transfer heat to liquid circulated through a heat exchanger.
2. Description of the Prior Art
Furnace design in general is a relatively old although the development of pool or spa heaters is relatively recent. Due to the increasing cost of fuels, designers of heaters or furnaces have become more concerned with increasing efficiency in their designs. Increasing emphasis has also been placed upon reducing the emission of pollutants, including unburned hydrocarbons, which may contaminate the environment and produce unpleasant odors. The present invention addresses the requirements of increased pool heater efficiency and the reduced smoke and odor through use of an improved insulated firebox.
The elimination of smoke from furnaces and incinerators has been the subject matter of several recent patents. U.S. Pat. Nos. 4,759,298 and 4,970,969 to Koptis et al. refer to the elimination of smoke released from a pyrolysis furnace. A pyrolysis furnace is used to reclaim and recycle metal parts that contain organic materials. The furnace volatilizes and burns the organic material which is bound to the metal parts. The volatiles are also burned in an afterburner chamber in the described furnaces. The furnace utilizes a thermocouple and water spray to maintain the combustion in the furnace within safe limits.
U.S. Pat. No. 4,553,488 to Giuseppe relates to a procedure and furnace for incineration of waste material without producing smoke. The incinerator has a steel cylindrical body and a heated plate which presses upon the material to be consumed. Heat is transferred from the heated plate to the waste material which is carbonized, distilled and ignited. Air is introduced under pressure to further achieve and sustain combustion and to avoid the formation of smoke.
In pool or spa heaters, a combustion chamber is commonly located inside a firebox. Insulation is used to define the firebox surrounding the combustion chamber to increase the heater's efficiency and also to reduce the exterior temperature of the heater. One problem with such insulation, however, is that organic materials in the insulation may produce smoke during start-up or continued operation of the heater. For example, insulation containing organic material heated to a temperature range of about 200.degree.-300.degree. F. may generate smoke or odor during start-up as a result of incomplete combustion of the organic material at these lower temperatures.
Also, for insulated fireboxes, a temperature gradient will exist in the insulation, causing the insulation temperature to decrease as the distance from the combustion chamber increases. When a fire is present, organic constituents in the combustion chamber will burn without generating smoke. However, the insulation farther from the combustion chamber will be at a lower temperature, and organic material that is present may create smoke.
Commonly this smoke escapes into the atmosphere and may become visible to those using the heater. Some particles of the smoke may deposit upon or in the vicinity of the heater. Furthermore, smoke produced at lower temperatures has a very pungent odor which may be objectionable to and may alarm users of the heater. This production of pungent smoke by pool or spa heaters and the deposition of particles in the vicinity of and upon the heater may be particularly noticeable if the heater is installed in an enclosed area. Installation of the pool or spa heater in a shed or pool-house may trap the smoke or odor and allow it to concentrate, thereby exacerbating the problem.